Method for humidifying boards of wood fibre or similar material



ug. 15, 1967 BENGT-KE LARssoN 3,335,499

METHOD FOR HUMIDIFYING BOARDS OF WOOD FIBRE OR SIMILAR MATERIAL Filed June 23, 1964 FIGI.

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United States Patent O 3,335,499 METHOD FR HUMIDIFYING BOARDS F WOOD FIBRE OR SIMILAR MATERIAL Bengt-ke Larsson, Farsta, Sweden, assignor to Aktiebolaget Svenska Flaktfabriken, Stockholm, Sweden Filed June 23, 1964, Ser. No. 377,338 Claims priority, application Sweden, Sept. 3, 1963, 9,642/63 1 Claim. (Cl. 34-13.8)

The present invention relates to a method which, in the humidifying of iboards of wood bre or -similar material by means of moist air, will ensure a uniform distribution of moisture in the boards.

Humidifying of iibrous material, for example sheets of wallboard Iand similar material, has hitherto been carried out with moist air in streams parallel to the mate-rial. Moist air ha-s been passed across the material in some cases and along the material in other cases. In both cases a substantial drop in the relative humidity of the air used for humidifyin-g will occur in the direction of air flow, which will cause ya difference in the humidity of the board on supply and exhaust sides. In other words, the moisture absorption of the board will travel forward like a wave, and a relatively long time must -be allowed before the board has attained an approximately uniform moisture ratio throughout.

This diiierence arising in the humidity of the air is the cause of other disadvantages, especially as -regard swelling, bulging, alteration in thickness etc., which are associated with an uneven supply of moisture.

The transmission` coeficient of diffusion is of great importance in the humidifying of material. An analogy exists between the transmission coefficient of diffusion and the heat transmission coefhcient a, usually expressed by Lewis law where Fy=density of the air and cp the specific heat. The constant k is dependent on the kin-d of flow and the state of the air. In other words it can be said that a high value of a will be accompanied by a high value. Under certain conditions, it can therefore =be said that the higher the value of a (val-ue of the shorter will be the time required to impart a certain desired moisture ratio, for example to a wallboard.

The object of the invention is to eliminate the said disadvantages and to permit a considerable reduction in the treatment time and to ensure a uniform moisture distribution in the finished boards.

The invention is characterized in that the moist air is supplied in a saturated or practically sat-urated state and in the form of evenly spaced jets with a high velocity and is blown against the boards at right angles against both sides.

Humidifying of wood fibre Iboards involves a swelling. It is a fact that this swelling is greatest during the beginning of humidifying; it is intensive up to a moisture ratio of 3 to 4%, and subsequently proceeds -relatively slowly. An intensive humidifying causes an erection of the iibres in the outer layer of the board, depending on stresses arising there due to the said swelling. When humidifying in air at 60 to 65 C., the erection of fibres is very small or does not take place at all; at higher temperatures, for example 90 C., it is violent. The time taken to humidify a board to a certain percentage is approximately twice as long at 60 C. than it is at 90 C.

In order to reduce the humidifying time and simultaneously avoid erection of the fibres, according to a practical embodiment of the method, the procedure can be such that the hurnidifying takes place in at least two tri-bution boxes, the walls of which -facin-g ice stages, separated by an interval, the temperature during the Ifirst sta-ge being maintained at approximately 606S C. and the temperature in the final stage at approximately t0-85 C.

The invention will be described in more detail with reference to the accompanying drawing wherein:

FIG. 1 is a diagram illustrating the method of humidifying in accordance with the present invention;

FIG. 2 is a fragmentary side view of a humidifying apparatus in accordance with the present invention;

FIG. 3 is a sectional view taken on the line 3-3 of FIG. 2; and

FIG. 4 is a thumbnail sketch in side elevation of apparatus for continuously humidifying in accordance with the present invention.

The method of the invention is illustrated in the accompanying diagram, FIG. l. The diagram shows the approximate relationship between moisture ratio (u) and time (tid) taken for a humidifying process according to the invention. During t-he time O-A the iboard is subjected to fairly moderate `air conditions with regard to the erection of libres, for example, air at a temperture of 60 C. and a relative humidity of The board is humidifed here to 3 to 4%. During the humidifying of the board, heat will be generated, partly due to condensation in the fibres, partly because the heat of swelling is released. The simplest way of removing this heat -is by ventilation otf the apparatus being used. In the diagram this has been indicated by the interval of time A-B. During the following interval B-C the board is humidied at a higher temperature until the desi-red humidity is reached for example, by air at a temperature of 80 C. an-d a relative humidity of During the interval A-B, vent-ilation can be combined with a cooling of the board, which helps to reduce the time B-C.

Humidifying according to the invention can be carried out either continuously or in batches.

A `continuous 'apparatus for carrying out the method is presupposed to consist of a treatment channel provided wit'h elements for the transport of the boards and equipped with elements for h-umidifying andY circulation of the treatment medium. The apparatus is characterized in that on both sides of each transport plane there are air disthe transport plane are provided with a large number of equally spaced discharge openings for blowing on treatment medium in the form of jets -directed substantially at righ-t angles to the boa-rds, and that the treatment channel is divided up by means of partitions into two or more effective treatment zones with intermediate equalizing zones, the latter being provided with outlets for exhausting ventilization air for the purpose of removing of excess heat generated in the channel.

Thus the invention is based on the realization that the conditions essential for obtaining a uniform distribution of moisture are that the treatment medium be supplied uniformly distributed and in such a manner that a high heat transmission coeflicient and transmission coefficient of diffusion are attained. According to the invention, this is achieved by utilization of moist air in a substantially saturated state blown perpendicularly against both sides of the board.

In addition to the advantage of a first-class linished product-With an absolutely uniform moisture ratiogains are ma-de in that initial costs are considerably reduced and the need for extra space for moisture equalization after humidifying is eliminated and in that the boards can proceed for further treatment without delay.

An apparatus for carrying the invention into practice will now be -described with reference to FIGS. 2 to 4 inclusive. In the gures, 1 designates a foundation on which the section 2 of a continuously-operating humidifying apparatus is supported. 3 designates circulating fans driven by means of motors 4 and belt drives. The circulating fans 3 lare arranged in pairs on opposite sides in order to ob- -tain .a uniform air distribution in the apparatus. A series of humidifying chambers 5 are arranged in the foundation 1, suitably one for each section. These are provided, in a conventional manner, with banks of nozzles 6, `drain gratings 7, a float valve 8 for supply of makeup water, a drain 9 for recirculating water, and a overflow 10. A number of blower boxes 11 are located in the sections situ-ated above the foundation. The horizontal Walls of lthese boxes could be perforated or provided with slots for right-angle `discharge of air. Conveyors 12 are arranged between the blower boxes. The 'boards to -be humidied 13 are transported on these conveyors. The conveyors can possibly be tted with double rollers to prevent lateral movement of the boards during transport. Air introduced into the boxes 11 through openings 14 in the right hand side of the apparatus is \orced out through perforations 1S on to the boards, to which it gives up moisture. T he air is then passed sideways out to the left hand side of the apparatus and is blown by fan 3 through the humidifying chamber, where it is again humidied. Since the apparatus is divided up into a number of sections and the quantity of circulating air is very large, the alteration in the relative humidity of the air will be very small, both .in the lengthwise direction of the apparatus and crosswise, which provides a uniform distribution of moisture throughout the entire board, and to Aa great extent helps to shorten the time required for humidifying. As shown in FIG. 4, the apparatus is divided into a irst treatment Zone 21, a second treatment zone 22, and an intermediate equalizin-g zone 23, all separated by partitions. The equalizing zone 23 is provided with an outlet 24 for exhausting Ventilating air to remove excess heat generated in the previous treatment zone 21.

The condition of the air in the respective zones and humiditying chambers is controlled -by conventional temperature control devices (not shown), for example, heat exchangers of the type disclosed in prior Patent Number 2,920,398; 2,939,222; or 3,070,902. The passage of the heated air through the humidifying chambers insures substantial saturation of the air owing therethrough.

What I claim is:

A method of humidifying boards of wood fibre or similar material which are subject to swelling at the start of humidiiication and erection of the bres in the outer lay er ofthe boards, wherein moist air is blown at right angles against -both sides of the board at high velocity and in the form of evenly spaced jets, the improvement wherein erection of the libres is avoided by `humidity-ing the boards in at least two stages with heated air which is in a substantially saturated state, and the stages are separated by a period of rest in which the boards are ventilated and cooled to remove the heat generated during humidication of the boards in the iirst stage d-ue to condensation in the fibre and Ithe heat of swelling, the temperature in the irst stage being maintained at approximately -65 C., and the temperature in the final stage being maintained at approximately -85 C.

References Cited UNITED STATES PATENTS 1,593,598 7/1926 Redman 34-13.4 1,668,314 5/1928 Harvey 34-205 X 2,906,034 9/1959 Robinson et al. 34-26 2,919,492 1/1960 Story 34--26 3,199,213 '8/1965 Milligan et al. 34-13.8 3,226,839 1/1966 Monaghan 34-21 FREDERICK L. MA'ITESON, IR., Primary Examiner.

I AMES W. WESTHAVER, Examiner.

A. D. HERRMANN, Assistant Examiner. 

